GSUP: introduce new messages for SS/USSD payloads

In order to be able to transfer SS/USSD messages via GSUP,
this change introduces the following new message types:

  - OSMO_GSUP_MSGT_PROC_SS_*,

and the following new IE:

  - OSMO_GSUP_SS_INFO_IE

which represents an ASN.1 encoded MAP payload coming to/from
the mobile station 'as is', without any transcoding.

Change-Id: Ie17a78043a35fffbdd59e80fd2b2da39cce5e532
Related: OS#1597
diff --git a/tests/gsup/gsup_test.ok b/tests/gsup/gsup_test.ok
index 1f59902..d63dd2d 100644
--- a/tests/gsup/gsup_test.ok
+++ b/tests/gsup/gsup_test.ok
@@ -29,4 +29,8 @@
           Send Authentication Info Request with AUTS and RAND (UMTS) OK
   Testing Dummy message with session IEs
           Dummy message with session IEs OK
+  Testing SS/USSD processUnstructuredSS-Request / Invoke
+          SS/USSD processUnstructuredSS-Request / Invoke OK
+  Testing SS/USSD processUnstructuredSS-Request / ReturnResult
+          SS/USSD processUnstructuredSS-Request / ReturnResult OK
 Done.